Field of the Invention
The frothing of a reactive component mix to produce a foamed product has been practiced heretofore by introducing a liquified, low-boiling propellant into reactive components which are injected under high pressure into a mixing chamber of a mixing head, causing them to impinge and thereby effect uniform mixing. Immediately upon emergence of the mix from the head, and even before any appreciable reaction between the components has occurred, there is an expansion due to partial vaporization of the liquified propellant with the reduction in pressure, producing a "pre-foamed" creamy product termed "froth".
The remainder of the propellant only evaporates at a high temperature, at which also the main components react. As a result there is a further expansion of the "pre-foamed" product to form a solid foamed material. The use of the "frothing" method accordingly divides up the foaming process into two stages.
The particular difficulty with this method is to be found in the mixing of the propellant with the remaining components which must be carried out at a sufficiently high pressure to effect adequate mixing. A method has already been proposed in the case of which the propellant is injected directly upstream from the mixing head into the supply line of one of the main components. A disadvantage in this respect is, however, the fact that recycling in periods between shots is impossible, since there is as a result an enrichment of propellant in the supply container for the respective main component, for example for the polyol.
A direct injection of the propellant into the mixing head is not possible either, since low pressure injection is impossible in view of the high pressure obtaining in the mixing chamber, and high pressure injection, owing to the low boiling point of the propellant, leads to an almost explosive expansion.
Injection of the propellant into the direct extension of the mixing chamber is not possible either, since this would lead to an excessively high acceleration of the component flow which is already mixed. The mixing in of the propellant would accordingly be made more difficult and the speed of emergence, limited in view of casting or injection considerations, would be exceded. It is not possible to make the mixing chamber as a whole larger for technical reasons concerned with the mixing operation.